Separation methods for water treatment include a method using a filtering membrane, a method using heat or phase-change, and so on. A separation method using a filtering membrane has a lot of advantages over the method using heat or phase-change. Among the advantages is the high reliability of water treatment since the water of desired purity can be easily and stably obtained by adjusting the size of the pores of the filtering membrane. Furthermore, since the separation method using a filtering membrane does not require a heating process, the membrane can be used with microorganisms which are useful for separation process but might be adversely affected by heat.
Among the separation methods using a filtering membrane is a method using a hollow fiber membrane module comprising a bundle of hollow fiber membranes. Typically, a hollow fiber membrane module has been widely used in the field of microfiltration and/or ultrafiltration for obtaining axenic water, drinking water, super pure water, and so on. Recently, the application of the hollow fiber membrane module is extended to wastewater treatment, solid-liquid separation in a septic tank, removal of suspended solid (SS) from industrial wastewater, filtration of river, filtration of industrial water, filtration of swimming pool water, and the like.
A hollow fiber membrane module may be classified into a submerged-type module and a pressurized-type module according to the operation manner thereof.
The submerged-type module performs the filtration process while immersed in fluid to be purified. Particularly, as negative pressure is applied inside the hollow fiber membrane, only pure fluid is allowed to penetrate the membrane and come into the lumen thereof while the contaminants such as impurities or sludge remains outside the membrane. The submerged-type module is advantageous in that it can decrease the costs for setting up the facilities and operating thereof since it does not require fluid circulation. It is disadvantageous, however, in that its permeation flux that can be obtained per unit time is relatively low.
On the other hand, during a filtration process by a pressurized-type module, a positive pressure is applied to the fluid outside the hollow fiber membrane so as to allow only pure fluid to penetrate the membrane and come into the lumen thereof. Although the pressurized-type module requires additional equipments for fluid circulation, it facilitates relatively high permeation flux per unit time, as compared to the submerged-type module.
Since the pressurized fluid is introduced into the body case of the pressurized-type module, the pressurized-type module is required to have some degree of pressure resistance. The conventional pressurized-type modules, however, exhibit the pressure resistance of at most 3 kgf/cm2.
Thus, as shown in FIG. 1, if the conventional pressurized-type module is used for a filtration system which carries out the first filtration process by means of a pressurized-type hollow fiber membrane module 10 and the second filtration process by means of a reverse osmosis membrane module 30, the first filtrate produced by the pressurized-type hollow fiber membrane module 10 needs to be stored in a filtrate bath 20 first because the first filtrate cannot be a sufficiently pressurized one due to the relatively low pressure resistance of the pressurized-type hollow fiber membrane module 10. Further, it is required to provide a feeding pump P1 to supply the first filtrate stored in the filtrate bath 20 toward the reverse osmosis membrane module 30. The first filtrate supplied toward the reverse osmosis membrane module 30 by the feeding pump P1 is pressurized by a high-pressure pump P2 with a pressure high enough to operate the reverse osmosis membrane module 30.
Hereinafter, the structural reason why the conventional pressurized-type hollow fiber membrane module 10 cannot have the pressure resistance higher than 3 kgf/cm2 will be described in detail with reference to the FIGS. 2 and 3. The FIG. 2 is a perspective view of the conventional pressurized-type hollow fiber membrane module, and the FIG. 3 is a cross-sectional view of the pressurized-type hollow fiber membrane module of FIG. 2 along the I-I′ line.
As illustrated in FIG. 2 and FIG. 3, a conventional pressurized-type hollow fiber membrane module 10 comprises a body case 11 having an open end, a fixing member 12 disposed in and fixed to the body case 11 thereby closing the open end, hollow fiber membranes 13 with one ends potted in the fixing member 12, a cap 14 on the open end of the body case 11, the cap 14 and fixing member 12 forming a filtrate-collecting space S, and a fastening ring 15 which, when coupled to the body case 11 through a screw-type coupling manner, pushes the cap 14 toward the body case 11 thereby tightly connecting the body case 11 and the cap 14 together.
The lumens of the hollow fiber membranes 13 are in fluid communication with the filtrate-collecting space S. It is only pure water among the feed water introduced in the module 10 through the inlet IH of the body case 11 that penetrates the hollow fiber membranes 13. The filtrate flows along the lumens of the hollow fiber membranes 13, gets together in the filtrate-collecting space S, and then comes out of the module 10 through the outlet OH of the cap 14. Meanwhile, as the filtration is performed, the impurity concentration of the feed water in the body case 11 increases. The condensed water produced as such is discharged from the module 10 through the outlet OH of the body case 11.
As illustrated in FIG. 3, the whole side surface of the fixing member 12 is in contact with the inner surface of the body case 11, and the gap G between the body case 11 and cap 14 is exposed to the filtrate-collecting space S. To prevent the leakage of the filtrate in the filtrate-collecting space S through the gap G, there is provided a sealing member 16 between the top surface of the body case 11 and the bottom surface of cap 14.
However, as long as the gap G between the body case 11 and cap 14 is exposed to the filtrate-collecting space S, there cannot but exist a limitation in preventing the leakage of the filtrate by means of the sealing member 16. Particularly, when the conventional pressurized-type hollow fiber membrane module 10 is operated with a pressure higher than 3 kgf/cm2, the pressure of the filtrates in the filtrate-collecting space S also increases and thus the leakage of the filtrate through the gap G between the body case 11 and cap 14 is inevitably caused.